A new series of N(6)-[(hetero)aryl/(cyclo)alkyl-carbamoyl-methoxy-phenyl]-(2-chloro)-5'-N-ethylcarboxamido-adenosines (24-43) has been synthesised and tested in binding assays at hA(1), hA(2A) and hA(3) adenosine receptors, and in a functional assay at the hA(2B) subtype. The examined compounds displayed high potency in activating A(2B) receptors with good selectivity versus A(2A) subtypes. The introduction of an unsubstituted 4-[(phenylcarbamoyl)-methoxy]-phenyl chain at the N(6) position of 5'-N-ethylcarboxamido-adenosine led us to the recognition of compound 24 as a full agonist displaying the highest efficacy of the series (EC(50) hA(2B)=7.3 nM). These compounds represent the first report about adenosine-related structures capable of activating hA(2B) subtype in the low nanomolar range.